Amyloid enhancing factor (AEF) is one of the factors in amyloidogenesis, besides the increase of amyloid precursor protein in a serum.We present the following results;1. We extracted AEF from organs of amyloidotic mice and the highest AEF activity was found in a spleen. On a histochemical study using anti-AEF antiserum, derived from the crude AEF, AEF is localized on neutrophils of spleen and peripheral blood.Electron microscopy reveals that AEF is not distributed in granules but in the cytosol of a neutrophil. Furthermore collected neutrophils are separated into granular and extra-granular fractions, with adding a degranulating agent, and only the granular fraction has AEF activity on a trial study on mice.2. AEF is purified with a DEAE ion exchange column and a reverse phase HPLC from neutrophils of spleen and peripheral blood treated with amyloidotic stimuli. AEF activity is in a fraction eluted with 0.17M saline on DEAE column and in a peak of retention time 23 min. on HPLC. SDS-page gives single band, about 15 KDa. considering with a result from a Gas-chromatography AEF is a glycoprotein weighing 15 KDa.3. Kupffer cells, cultured in the media with AEF, have many cell processes and swell, and on electron microscopic observation they indicate degenerations of organelle.These results suggest that AEF may cause damage on macrophages and then amyloid precursor protein increased with amyloidotic stimuli, which is not able to be degradated, is polymerized and changes into fibrils.